The prevalence of nonalcoholic fatty liver disease (NAFLD), the hepatic manifestation of the metabolic syndrome, is reaching epidemic proportions in the United States. Inflammation and fibrosis are the key factors heralding the progression from steatosis (fat accumulation in the liver) to nonalcoholic steatohepatitis (NASH), yet there has been a gap in understanding the molecular mechanisms underlying this transition. Numerous studies have shown that patients with NASH have increased circulating levels and hepatic expression of plasminogen activator inhibitor-1 (PAI-1), however it is unknown whether this association is causative or simply correlative. I intend to demonstrate that PAI-1, a factor primarily known for its role in thrombosis, also plays a major role in hepatic steatosis, inflammation, and fibrosis. Work in our lab and others has shown that PAI-1 transcription is strongly regulated by the molecular clock. Furthermore, disruption of circadian rhythms promotes the development of metabolic diseases including NAFLD. Therefore, the goal of this proposal is to determine a novel mechanism by which circadian gene disruption and upregulation of hepatic PAI-1 promote hepatic steatosis and steatohepatitis. In specific aim 1, we will determine the role of PAI-1 in the development of murine steatohepatitis. First we will determine the effects of hepatocyte-derived PAI-1 on murine steatohepatitis by generating a novel murine model of hepatocyte-specific PAI-1 deletion. Next, we will determine the role of bone-marrow derived PAI-1 on murine steatohepatitis using bone-marrow chimeric mice. Lastly we will determine the effects of a novel pharmacologic inhibitor of PAI-1 on the development of murine steatohepatitis. The results of the experiments outlined in Aim 1 will (i) implicate PAI-1 as a key mediator of murine steatohepatitis, (ii) determine the cell-type from which the effects of PAI-1 are derived, and (iii) identify PAI-1 antagonists as a potential novel pharmacotherapy for NASH. In specific aim 2, we will test the hypothesis that disruption of the circadian clock promotes hepatic steatosis via activation of hepatic PAI-1. First we will test the effect of altere feeding cycles (a method of inducing circadian dyssynchrony) on hepatic lipid accumulation in PAI-1-deficient mice. Next, we will test the effects of genetic and pharmacologic manipulation of PAI-1 on the steatotic phenotype observed in mice bearing a mutation in a key clock gene. The results of the experiments outlined in Aim 2 will identify PAI-1 as the critical link between disruption in circadian rhythms and hepatic lipid deposition. These studies represent the progression in my long- standing research on the molecular pathogenesis of steatohepatitis and provide the platform for my development as an independent physician-scientist.